There must be a common basis.

You can't even argue with somebody, or disagree with somebody unless you have a common ground to start from.

If I say that black is white, and you think it is blue, we can't get on.

It leads nowhere.

And, finally--" She turned round and faced him directly.
"Finally, don't be so cross, my dear," she said.
"But am I ?" asked he.
"Yes.